Interactionism

Interactionism is the view that mind and body—or mental events and physical events—causally influence each other. That this is so is one of our common-sense beliefs, because it appears to be a feature of everyday experience. The physical world influences my experience through my senses, and I often react behaviourally to those experiences. My thinking, too, influences my speech and my actions. There is, therefore, a massive natural prejudice in favour of interactionism. It has been claimed, however, that it faces serious problems (some of which were anticipated in section 1).
The simplest objection to interaction is that, in so far as mental properties, states or substances are of radically different kinds from each other, they lack that communality necessary for interaction. It is generally agreed that, in its most naive form, this objection to interactionism rests on a ‘billiard ball’ picture of causation: if all causation is by impact, how can the material and the immaterial impact upon each other? But if causation is either by a more ethereal force or energy or only a matter of constant conjunction, there would appear to be no problem in principle with the idea of interaction of mind and body.
Even if there is no objection in principle, there appears to be a conflict between interactionism and some basic principles of physical science. For example, if causal power was flowing in and out of the physical system, energy would not be conserved, and the conservation of energy is a fundamental scientific law. Various responses have been made to this. One suggestion is that it might be possible for mind to influence the distribution of energy, without altering its quantity. (See Averill and Keating 1981). Another response is to challenge the relevance of the conservation principle in this context. The conservation principle states that ‘in a causally isolated system the total amount of energy will remain constant’. Whereas ‘[t]he interactionist denies…that the human body is an isolated system’, so the principle is irrelevant (Larmer (1986), 282: this article presents a good brief survey of the options).
Robins Collins (2011) has claimed that the appeal to conservation by opponents of interactionism is something of a red herring because conservation principles are not ubiquitous in physics. He argues that energy is not conserved in general relativity, in quantum theory, or in the universe taken as a whole. Why then, should we insist on it in mind-brain interaction?
Most discussion of interactionism takes place in the context of the assumption that it is incompatible with the world’s being ‘closed under physics’. This is a very natural assumption, but it is not justified if causal overdetermination of behaviour is possible. There could then be a complete physical cause of behaviour, and a mental one. The strongest intuitive objection against overdetermination is clearly stated by Mills (1996: 112), who is himself a defender of overdetermination.
For X to be a cause of Y, X must contribute something to Y. The only way a purely mental event could contribute to a purely physical one would be to contribute some feature not already determined by a purely physical event. But if physical closure is true, there is no feature of the purely physical effect that is not contributed by the purely physical cause. Hence interactionism violates physical closure after all.
Mills says that this argument is invalid, because a physical event can have features not explained by the event which is its sufficient cause. For example, “the rock’s hitting the window is causally sufficient for the window’s breaking, and the window’s breaking has the feature of being the third window-breaking in the house this year; but the facts about prior window-breakings, rather than the rock’s hitting the window, are what cause this window-breaking to have this feature.”
The opponent of overdetermination could perhaps reply that his principle applies, not to every feature of events, but to a subgroup—say, intrinsic features, not merely relational or comparative ones. It is this kind of feature that the mental event would have to cause, but physical closure leaves no room for this. These matters are still controversial.
The problem with closure of physics may be radically altered if physical laws are indeterministic, as quantum theory seems to assert. If physical laws are deterministic, then any interference from outside would lead to a breach of those laws. But if they are indeterministic, might not interference produce a result that has a probability greater than zero, and so be consistent with the laws? This way, one might have interaction yet preserve a kind of nomological closure, in the sense that no laws are infringed. Because it involves assessing the significance and consequences of quantum theory, this is a difficult matter for the non-physicist to assess. Some argue that indeterminacy manifests itself only on the subatomic level, being cancelled out by the time one reaches even very tiny macroscopic objects: and human behaviour is a macroscopic phenomenon. Others argue that the structure of the brain is so finely tuned that minute variations could have macroscopic effects, rather in the way that, according to ‘chaos theory’, the flapping of a butterfly’s wings in China might affect the weather in New York. (For discussion of this, see Eccles (1980), (1987), and Popper and Eccles (1977).) Still others argue that quantum indeterminacy manifests itself directly at a high level, when acts of observation collapse the wave function, suggesting that the mind may play a direct role in affecting the state of the world (Hodgson 1988; Stapp 1993).